Again, I've seen several threads on this and I'd just like to consolidate them in one place.
At this point, we do know there's a conflict with BBPress that can cause some fairly bad errors on activation of NextGEN 2.0. If you happen to be reading this prior to updating, you should hold off.
If you've already updated, the best thing you can do right now is roll back to 1.9.13. See our roll back guide here:
In any case, if you are having an error or conflict with BBPress, can you please post your details here. You can also sign up to get notified of follow-ups on this post if you want to stay up-to-date.
We're not sure what the fix will be for this, but it's fairly high priority.